Rechargeable battery with resistive layer for enhanced safety

ABSTRACT

An improved high energy density rechargeable (HEDR) battery with an anode energy layer, a cathode energy layer, a separator between the anode and cathode energy layers for preventing internal discharge thereof, and at least one current collector for transferring electrons to and from either the anode or cathode energy layer, includes a resistive layer interposed between the separator and one of the current collectors for limiting the rate of internal discharge through the failed separator in the event of separator failure. The resistive layer has a fixed resistivity at temperatures between a preferred temperature range and an upper temperature safety limit for operating the battery. The resistive layer serves to avoid temperatures in excess of the upper temperature safety limit in the event of separator failure in the battery, and a fixed resistivity of the resistive layer is greater than the internal resistivity of either energy layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to thefollowing three Provisional Applications: U.S. Provisional ApplicationNo. 62/084,454, filed Nov. 25, 2014, titled “Battery Safety Device;”U.S. Provisional Application No. 62/114,001, filed Feb. 9, 2015, titled“Rechargeable Battery with Resistive Layer for Enhanced Safety;” andU.S. Provisional Application No. 62/114,508, filed Feb. 10, 2015, titled“Rechargeable Battery with Internal Current Limiter and Interrupter,”the disclosures of which are all hereby incorporated by referenceherein, each in its entirety.

BACKGROUND

1. Technical Field

This disclosure relates to an internal current limiter or currentinterrupter used to protect a battery in the event of an internal shortcircuit leads to thermal runaway. In particular, it relates to a highenergy density rechargeable (HEDR) battery with improved safety.

2. Background

There is a need for rechargeable battery systems with enhanced safetythat have a high energy density and hence are capable of storing anddelivering large amounts of electrical energy per unit volume and/orweight. Such stable high-energy battery systems have significant utilityin a number of applications including military equipment, communicationequipment, and robotics.

An example of a high energy density rechargeable (HEDR) battery commonlyin use is the lithium-ion battery.

A lithium-ion battery is a rechargeable battery wherein lithium ionsmove from the negative electrode to the positive electrode duringdischarge and back when charging. Lithium-ion batteries can be dangerousunder some conditions and can pose a safety hazard. The fire energycontent (electrical+chemical) of lithium cobalt-oxide cells is about 100to 150 kJ/A-h (kilojoules per Amp-hour), most of it chemical. Ifovercharged or overheated, Li-ion batteries may suffer thermal runawayand cell rupture. In extreme cases this can lead to combustion. Also,short-circuiting the battery, either externally or internally, willcause the battery to overheat and possibly to catch fire.

Overcharge:

In a lithium-ion battery, useful work is performed when electrons flowthrough a closed external circuit. However, in order to maintain chargeneutrality, for each electron that flows through the external circuit,there must be a corresponding lithium ion that is transported from oneelectrode to the other. The electric potential driving this transport isachieved by oxidizing a transition metal. For example, cobalt (Co), fromCo³⁺ to Co⁴⁺ during charge and reduced from Co⁴⁺ to Co³⁺ duringdischarge. Conventionally, Li_(1−χ)CoO₂ may be employed, where thecoefficient χ represents the molar fraction of both the Li ion and theoxidative state of CoO₂, viz., Co³⁺ or Co⁴⁺. Employing theseconventions, the positive electrode half-reaction for the lithium cobaltbattery is represented as follows:

LiCoO₂

Li_(1−χ)CoO₂+χLi⁺ +χe ⁻

The negative electrode half reaction is represented as follows:

χLi⁺ +χe ⁻+χC₆

χLiC₆

The cobalt electrode reaction is reversible limited to χ<0.5, limitingthe depth of discharge allowable because of cycle life considerationsand the stability of LiCoO₂. Overcharge leads to the synthesis ofcobalt(IV) oxide, as follows:

LiCoO₂→Li⁺+CoO₂+O₂ +e ⁻

LiCoO₂ will decompose into CoO₂ and release a large amount of heat andoxygen. The released oxygen will then oxidize the electrolyte, whichwill lead to thermal runaway. This process is irreversible. Therefore,what is needed is some device or design that can decompose below orbefore positive decomposition. This device will protect the cell fromthermal runaway.

Thermal Runaway:

If the heat generated by a lithium ion battery exceeds its heatdissipation capacity, the battery can become susceptible to thermalrunaway, resulting in overheating and, under some circumstances, todestructive results such as fire or violent explosion. Thermal runawayis a positive feedback loop wherein an increase in temperature changesthe system so as to cause further increases in temperature. The excessheat can result from battery mismanagement, battery defect, accident, orother causes. However, the excess heat generation often results fromincreased joule heating due to excessive internal current or fromexothermic reactions between the positive and negative electrodes.Excessive internal current can result from a variety of causes, but alowering of the internal resistance due to separator short circuit isone possible cause. Heat resulting from a separator short circuit cancause a further breach within the separator, leading to a mixing of thereagents of the negative and positive electrodes and the generation offurther heat due to the resultant exothermic reaction.

Internal Short Circuit:

Lithium ion batteries employ a separator between the negative andpositive electrodes to electrically separate the two electrodes from oneanother while allowing lithium ions to pass through. When the batteryperforms work by passing electrons through an external circuit, thepermeability of the separator to lithium ions enables the battery toclose the circuit. Short circuiting the separator by providing aconductive path across it allows the battery to discharge rapidly. Ashort circuit across the separator can result from improper charging anddischarging or cell manufacturing defects such as metal impurities andmetal shard formation during electrode production. More particularly,improper charging can lead to the deposition of metallic lithiumdendrites on the surface of the negative electrode and grow to penetratethe separator through the nanopores so as to provide a conductive pathfor electrons from one electrode to the other. In addition, improperdischarge at or below 1.5V will cause copper dissolution that can leadto the formation of metallic copper dendrites on the surface of thenegative electrode that can also grow to penetrate the separator throughthe nanopore. The lower resistance of the conductive path allows forrapid discharge and the generation of significant joule heat.Overheating and thermal runaway can result.

An internal current limiter could limit the rate of internal dischargeresulting from an internal short circuit, including a short circuit ofthe separator, regardless of the temperature increase in a lithium ionbattery.

SUMMARY

It is disclosed herein that a high energy density rechargeable (HEDR)battery may usefully incorporate an internal non-sacrificial currentlimiter to protect the battery in the event of an internal short circuitor overcharge. The current limiter is a resistive film of fixedresistance interposed between the electrode or separator and currentcollector. The fixed resistance of the resistive film remains stablewhen the battery is overheated.

A first aspect of the disclosure is directed to an improved high energydensity rechargeable (HEDR) battery of a type including an anode energylayer, a cathode energy layer, a separator between the anode energylayer and the cathode energy layer for preventing internal dischargethereof, and at least one current collector for transferring electronsto and from either the anode or cathode energy layer. The anode andcathode energy layers each have an internal resistivity. The HEDRbattery has a preferred temperature range for discharging electriccurrent and an upper temperature safety limit. The improvement isemployable, in the event of separator failure, for limiting the rate ofinternal discharge through the failed separator and the generation ofjoule heat resulting therefrom. More particularly, the improvementcomprises a resistive layer interposed between the separator and one ofthe current collectors for limiting the rate of internal dischargethrough the failed separator in the event of separator failure. Theresistive layer has a fixed resistivity at temperatures between thepreferred temperature range and the upper temperature safety limit. Thefixed resistivity of the resistive layer is greater than the internalresistivity of either energy layer. The resistive layer helps thebattery avoid temperatures in excess of the upper temperature safetylimit in the event of separator failure.

In a first aspect, provided herein is a high energy density rechargeable(HEDR) battery that includes an anode energy layer, a cathode energylayer, a separator between the anode energy layer and the cathode energylayer for preventing internal discharge thereof, at least one currentcollector for transferring electrons to and from either the anode orcathode energy layer, the anode and cathode energy layers each having aninternal resistivity, the HEDR battery having a preferred temperaturerange for discharging electric current and an upper temperature safetylimit; and a resistive layer interposed between the separator and one ofthe current collectors, the resistive layer configured to limit the rateof internal discharge through the separator in the event of separatorfailure and the generation of joule heat resulting therefrom, theresistive layer having a fixed resistivity at temperatures between thepreferred temperature range and the upper temperature safety limit, thefixed resistivity of the resistive layer being greater than the internalresistivity of either energy layer, the resistive layer for avoidingtemperatures in excess of the upper temperature safety limit in theevent of separator failure.

The following features can be included in the HEDR battery in anysuitable combination. In some implementations, the resistive layer ofthe HEDR battery can be porous and include a ceramic powder defining aninterstitial space, a binder for partially filling the interstitialspace for binding the ceramic powder; and a conductive componentdispersed within the binder for imparting resistivity to the resistivelayer, the interstitial space remaining partially unfilled for impartingporosity and permeability to the resistive layer. The resistive layercan be compressed to reduce the unfilled interstitial space and increasethe binding of the ceramic powder by the binder. The resistive layer caninclude greater than 30% ceramic powder by weight. The resistive layercan include greater than 50% ceramic powder by weight. The resistivelayer can include greater than 70% ceramic powder by weight. Theresistive layer can include greater than 75% ceramic powder by weight.The resistive layer can include greater than 80% ceramic powder byweight. The resistive layer of the HEDR battery can be permeable totransport of ionic charge carriers. The resistive layer can benon-porous and have a composition that includes a non-conductive filler,a binder for binding the non-conductive filler, and a conductivecomponent dispersed within the binder for imparting resistivity to theresistive layer. The resistive layer can be impermeable to transport ofionic charge carriers. The fixed resistivity of the resistive layer ofthe HEDR battery can be at least twice as great as the internalresistivity of either energy layer. The fixed resistivity of theresistive layer can be at least five times as great as the internalresistivity of either energy layer. The fixed resistivity of theresistive layer can be at least ten times as great as the internalresistivity of either energy layer. The resistive layer can lack atransformation from solid phase to non-solid phase for transforming theresistivity of the resistive layer from low resistivity to highresistivity at temperatures between the maximum operating temperatureand the upper temperature safety limit. The resistive layer can benon-sacrificial at temperatures below the upper temperature safetylimit. The resistive layer can be sacrificial at temperatures above theupper temperature safety limit. The resistive layer can include aceramic powder that chemically decomposes above the upper temperaturesafety limit for evolving a fire retardant gas. The resistive layer caninclude a ceramic powder that chemically decomposes above the uppertemperature safety limit for evolving a gas for delaminating the currentcollector from the resistive layer. The current collector can include ananode current collector for transferring electrons to and from the anodeenergy layer, wherein the resistive layer is interposed between theseparator and the anode current collector. The resistive layer can beinterposed between the anode current collector and the anode energylayer. The resistive layer can be interposed between the anode energylayer and the separator. In some implementations, the anode energy layerof the HEDR battery can include a first anode energy layer, and a secondanode energy layer interposed between the first anode energy and theseparator, wherein the resistive layer is interposed between the firstanode energy layer and the second anode energy layer. The currentcollector can include a cathode current collector for transferringelectrons to and from the cathode energy layer, wherein the resistivelayer is interposed between the separator and the cathode currentcollector. The resistive layer can be interposed between the cathodecurrent collector and the cathode energy layer. The resistive layer canbe interposed between the cathode energy layer and the separator. Thecathode energy layer can include a first cathode energy layer, and asecond cathode energy layer interposed between the first cathode energyand the separator, wherein the resistive layer is interposed between thefirst cathode energy layer and the second cathode energy layer. In someimplementations, the HEDR battery can include two current collectorsthat include an anode current collector for transferring electrons toand from the anode energy layer, and a cathode current collector fortransferring electrons to and from the cathode energy layer in which theresistive layer comprises an anode resistive layer and a cathoderesistive layer, the anode resistive layer interposed between theseparator and the anode current collector, the cathode resistive layerinterposed between the separator and the cathode current collector.

In a related aspect, provided herein is a method for limiting the rateof an internal discharge of energy layers resulting from a separatorfailure within a high energy density rechargeable (HEDR) battery, themethod that includes resisting the internal discharge with a resistivelayer, the resistive layer being interposed between a separator and acurrent collector within the HEDR battery, the resistive layer having afixed resistivity at temperatures between a preferred temperature rangefor discharging the energy layers and an upper temperature safety limit,the fixed resistivity of the resistive layer being greater than theinternal resistivity of the energy layers.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the disclosure, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-1J illustrate schematic representations of exemplaryconfigurations of film-type lithium ion batteries having one or moreresistive layers serving as current limiters for protecting the batteryagainst overheating in the event of an internal short circuit.

FIGS. 2A-2E illustrate cross sectional views of prior art film-typelithium ion batteries (FIGS. 2A and B) and of film-type lithium ionbatteries with a resistance layer (FIGS. 2C and D).

FIGS. 3A-3E illustrate cross sectional views of prior art film-typelithium ion batteries

(FIGS. 3A and B) and of film-type lithium ion batteries of with aresistance layer (FIGS. 2C and D).

FIGS. 4A-4E illustrate cross sectional views of prior art film-typelithium ion batteries (FIGS. 4A and B) and of film-type lithium ionbatteries with a resistance layer (FIGS. 4C and D).

FIGS. 5A-5E illustrate cross sectional views of prior art film-typelithium ion batteries (FIGS. 5A and B) and of film-type lithium ionbatteries with a resistance layer (FIGS. 5C and D).

FIGS. 6A-6C illustrate exemplary structures for the resistive layer (8).

FIGS. 7A and 7B illustrate exemplified Cell compositions described inthe Examples below.

FIG. 8 illustrates the cell impedance and capacities at differentcurrents for Cells 1, 3, 4, 5, and 6 described in the Examples below.

FIG. 9 illustrates the resistance of Cell 2 (baseline, no resistivelayer) at 3.6V vs graphite in relation to the temperature increase. Theresistance decreased about 10 times with the increase in thetemperature.

FIG. 10 illustrates the resistance of Cell 3 at 4.09V vs graphite inrelation to the temperature increase. The resistance decreased slightlyand increased by about 3 times and then decreased by about 3 times withthe increase in the temperature.

FIG. 11 illustrates the discharge capacity of Cell 1 (baseline, no anyresistive layer) vs the cell voltage at the currents of 1 A, 3 A, 6 Aand 10 A.

FIG. 12 illustrates the discharge capacity of Cell 4 vs the cell voltageat 1 A, 3 A, 6 A and 10 A. The cell discharge capability decreasesdramatically with the increase in the cell discharge current with thisparticular resistive layer.

FIG. 13 illustrates the Impact Test design.

FIG. 14 illustrates the Cell temperature profiles during the impact testfor Cells 1, 3, 4, 5, and 6. All cells with the resistive layer passedthe test while the cell without any resistive layer failed in the test(caught on fire). The maximum cell temperature during the impact test issummarized in FIG. 15.

FIG. 15 illustrates the maximum temperature obtained by Cells 1, 3, 4,5, and 6 during the impact test.

FIG. 16 illustrates the cell voltage and temperature vs the impacttesting time for Cells 4 and 5. The impact starting time is set to zero.The cell voltage drop to zero voltage as soon as the cell is impacted.Then the cell temperature increases quickly. The temperature of the cellwith the resistive layer increases much slowly than that of Cell 1 (seeFIG. 14).

FIG. 17 illustrates the cycle life of Cell 3. The cell lost about 2%after 100 cycles which is similar to that of the cells without anyresistive layer (˜2.5% by average, not shown).

FIG. 18 shows cell temperature and overcharge voltage profiles during2A/12V overcharge test at room temperature.

DETAILED DESCRIPTION

Safe, long-term operation of high energy density rechargeable batteries,including lithium ion batteries, is a goal of battery manufacturers. Oneaspect of safe battery operation is controlling the discharge of theserechargeable batteries. As described above, a separator, or barrierlayer, is used to separate the negative and positive electrodes inrechargeable batteries in which ions can move through the battery, butelectrical current is forced to flow outside the battery, through anexternal circuit. Many factors may cause the separator to be breached,and may cause a short-circuit to occur within a rechargeable battery. Ashort-circuit leads to rapid discharge and possibly overheating andthermal runaway. Described below are apparatus and methods associatedwith an internal current limiter that limits the rate of internaldischarge in a rechargeable battery when there is an internal shortcircuit.

Described herein is an improved high energy density rechargeable (HEDR)battery that includes an anode energy layer, a cathode energy layer, aseparator between the anode energy layer and the cathode energy layerfor preventing internal discharge thereof, and at least one currentcollector for transferring electrons to and from either the anode orcathode energy layer. The anode and cathode energy layers can each havean internal resistivity. The HEDR battery can have a preferredtemperature range for discharging electric current and an uppertemperature safety limit. In the event of separator failure, a resistivelayer can be used for limiting the rate of internal discharge throughthe failed separator and the generation of joule heat resultingtherefrom. The resistive layer can be interposed between the separatorand one of the current collectors for limiting the rate of internaldischarge through the failed separator in the event of separatorfailure. The resistive layer can have a fixed resistivity attemperatures between the preferred temperature range and the uppertemperature safety limit. The fixed resistivity of the resistive layercan be greater than the internal resistivity of either energy layer. Theresistive layer can help the battery avoid temperatures in excess of theupper temperature safety limit in the event of separator failure.

In some embodiments, the resistive layer can be porous or not porous andhas a composition that includes a ceramic powder defining aninterstitial space, a binder for partially filling the interstitialspace for binding the ceramic powder, and a conductive componentdispersed within the binder for imparting resistivity to the resistivelayer. The interstitial space remains partially unfilled for impartingporosity and permeability to the resistive layer. The resistive layercan be compressed for reducing the unfilled interstitial space andincreasing the binding of the ceramic powder by the binder. Moreparticularly, the ceramic powder may have a weight percent of theresistive layer of about 30 to 99%; alternatively, the ceramic powdermay have a weight percent of the resistive layer of about 50 to 90%;alternatively, the ceramic powder may have a weight percent of theresistive layer of about 60 to 80%. The resistive layer may be permeableto transport of ionic charge carriers.

In some embodiments, the resistive layer can be non-porous and can havea composition that includes a non-conductive filler, a binder forbinding the non-conductive filler, and a conductive component dispersedwithin the binder for imparting resistivity to the resistive layer. Theresistive layer may be impermeable to transport of ionic chargecarriers.

In any of the embodiments of the battery, the fixed resistivity of theresistive layer may be at least twice as great as the internalresistivity of either energy layer; alternatively, the fixed resistivityof the resistive layer may be at least five times as great as theinternal resistivity of either energy layer; alternatively, the fixedresistivity of the resistive layer may be at least ten times as great asthe internal resistivity of either energy layer.

Furthermore, in any of the embodiments of the battery, the resistivelayer may lack a physical phase transformation at temperatures betweenthe preferred temperature range and the upper temperature safety limitfor transforming the resistivity of the resistive layer. Moreparticularly, the resistive layer may lack a transformation from solidphase to non-solid phase for transforming the resistivity of theresistive layer from low resistivity to high resistivity at temperaturesbetween the maximum operating temperature and the upper temperaturesafety limit. The resistive layer may be non-sacrificial at temperaturesbelow the upper temperature safety limit. However, the resistive layermay simultaneously be sacrificial at temperatures above the uppertemperature safety limit.

In some embodiments of the battery, the HEDR battery can be of a type inwhich the current collector includes an anode current collector fortransferring electrons to and from the anode energy layer. In theseembodiments, the resistive layer may be interposed between the separatorand the anode current collector. Alternatively, resistive layer may beinterposed between the anode current collector and the anode energylayer or, the resistive layer may be interposed between the anode energylayer and the separator.

In some embodiments of the battery, the HEDR battery can be of a type inwhich the anode energy layer includes a first anode energy layer and asecond anode energy layer interposed between the first anode energy andthe separator. In such embodiments, the resistive layer may beinterposed between the first anode energy layer and the second anodeenergy layer.

In some embodiments of the battery, the HEDR battery is of a type inwhich the current collector includes a cathode current collector fortransferring electrons to and from the cathode energy layer. In theseembodiments, the resistive layer may be interposed between the separatorand the cathode current collector. Alternatively, resistive layer may beinterposed between the cathode current collector and the cathode energylayer, or the resistive layer may be interposed between the cathodeenergy layer and the separator.

In some embodiments of the battery, the HEDR battery can be of a type inwhich the cathode energy layer includes a first cathode energy layer anda second cathode energy layer interposed between the first cathodeenergy and the separator. In these embodiments, the resistive layer maybe interposed between the first cathode energy layer and the secondcathode energy layer.

The HEDR battery can be of a type further having two current collectorsincluding an anode current collector for transferring electrons to andfrom either the anode energy layer and/or a cathode current collectorfor transferring electrons to and from either the cathode energy layer.In some embodiments, the resistive layer can include an anode resistivelayer and a cathode resistive layer. The anode resistive layer isinterposed between the separator and the anode current collector. Thecathode resistive layer is interposed between the separator and thecathode current collector.

A method for employing the current limiter (e.g. resistance layer) forlimiting the rate of an internal discharge of energy layers resultingfrom a separator failure within a HEDR battery. The method comprises thestep of resisting the internal discharge with a resistive layer. Theresistive layer is interposed between a separator and a currentcollector within the HEDR battery. The resistive layer has a fixedresistivity at temperatures between a preferred temperature range fordischarging the energy layers and an upper temperature safety limit. Thefixed resistivity of the resistive layer is greater than the internalresistivity of the energy layers.

FIGS. 1A-1J illustrate schematic representations of exemplaryconfigurations of film-type lithium ion batteries having one or moreresistive layers serving as current limiters for protecting the batteryagainst overheating in the event of an internal short circuit. FIGS. 1A,1B, 1D, and 1E show configurations each with a cathode current collector101, a cathode energy layer 102, a separator 103, one resistive layer108, an anode energy layer 104, and an anode collector 105. FIG. 1Cshows a configuration for a film-type lithium ion battery with a cathodecurrent collector 101, a cathode energy layer 102, a separator 103, aresistive layer 108, a first anode energy layer 106, a second anodeenergy layer 107, and an anode collector 105. FIG. 1F shows aconfiguration for a film-type lithium ion battery with a cathode currentcollector 101, a first cathode energy layer 109, a second cathode energylayer 110, a separator 103, a resistive layer 108, an anode energy layer104, and an anode collector 105. FIGS. 1G-1J show configurations eachwith a cathode current collector 101, a cathode energy layer 102, aseparator 103, two resistive layers 111 and 112, an anode energy layer104, and an anode collector 105.

FIGS. 2A-2E illustrate cross sectional views of prior art film-typelithium ion batteries (FIGS. 2A and B) and of film-type lithium ionbatteries with a resistance layer (FIGS. 2C and D). More particularly,FIGS. 2A-2E illustrate the current flow through film-type lithium ionbatteries undergoing discharge for powering a load (L). FIGS. 2A and Cillustrate the current flow of film-type lithium ion batteries having anintact fully operational separator (unshorted). FIGS. 2B and Dillustrate the current flow of film-type lithium ion batteries havingresistive layer serving as a current limiter, wherein the separator hasbeen short circuited by a conductive dentrite penetrating there through.In FIGS. 2B and D, the cells are undergoing internal discharge. Notethat devices with unshorted separators (FIGS. 2A and C) and the priorart device with the shorted separator (FIG. 2B), current flows from onecurrent collector to the other. However, in the exemplary device, shownin FIG. 2E, having a shorted separator and resistive layer 8 (FIG. 2D),current flow is diverted from the current collector and is much reduced.

FIGS. 3A-3E illustrate cross sectional views of prior art film-typelithium ion batteries (FIGS. 3A and B) and of film-type lithium ionbatteries of with a resistance layer (FIGS. 2C and D). Moreparticularly, FIGS. 3A-3E illustrate the current flow through film-typelithium ion batteries while its being charged by a smart power supply(PS) that will stop the charging when it detects any abnormal voltage.FIGS. 3A and C illustrate the current flow of film-type lithium ionbatteries having an intact fully operational separator (unshorted).FIGS. 3B and D illustrate the current flow of film-type lithium ionbatteries having a having a separator shorted by a conductive dentrite.Note that devices with unshorted separators (FIGS. 3A and C) and theprior art device with the shorted separator (FIG. 3B), current flowsfrom one current collector to the other. However, in the exemplarydevice, shown in FIG. 3E, having a shorted separator and resistive layer8 (as shown in FIG. 3D), current flow is diverted from the currentcollector and is much reduced.

FIGS. 4A-4E illustrate cross sectional views of prior art film-typelithium ion batteries (FIGS. 4A and B) and of film-type lithium ionbatteries with a resistance layer (FIGS. 4C and D). More particularly,FIGS. 4A-4E illustrate the current flow through film-type lithium ionbatteries undergoing discharge for powering a load (L). FIGS. 4A and Cillustrate the current flow of film-type lithium ion batteries having anintact fully operational separator (unshorted). FIGS. 4B and Dillustrate the current flow of film-type lithium ion batteries having ashort circuit caused by a disrupted separator. Note that devices withunshorted separators (FIGS. 4A and C) and the prior art device with theshorted separator (FIG. 4B), current flows from one current collector tothe other. However, in the exemplary device, shown in FIG. 4E, having ashorted separator and resistive layer 8 (as shown in FIG. 4D), currentflow is diverted from the current collector and is much reduced.

FIGS. 5A-5E illustrate cross sectional views of prior art film-typelithium ion batteries (FIGS. 5A and B) and of film-type lithium ionbatteries with a resistance layer (FIGS. 5C and D). More particularly,FIGS. 5A-5E illustrate the current flow through film-type lithium ionbatteries while its being charged by a smart power supply (PS) that willstop the charging when it detects any abnormal charging voltage. FIGS.5A and C illustrate the current flow of film-type lithium ion batterieshaving an intact fully operational separator (unshorted). FIGS. 5B and Dillustrate the current flow of film-type lithium ion batteries having ahaving a short circuit caused by a disrupted separator. Note thatdevices with unshorted separators (FIGS. 5A and C) and the prior artdevice with the shorted separator (FIG. 5B), current flows from onecurrent collector to the other. However, in the exemplary device, shownin FIG. 5E, having a shorted separator and resistive layer 8 (as shownin FIG. 5D), current flow is diverted from the current collector and ismuch reduced.

FIG. 6 illustrates exemplary structures for the resistive layer (8).FIG. 6A illustrates resistive layer having a high proportion of ceramicparticles (80% or more) coated with binder. Interstitial voids betweenthe coated ceramic particles render the resistive layer porous. FIG. 6Billustrates resistive layer having a high proportion of ceramicparticles (80% or more) bound together by particles of binder.Interstitial voids between the coated ceramic particles render theresistive layer porous. FIG. 6C illustrates resistive layer having anintermediate proportion of ceramic particles (less than 80%) heldtogether with binder. The resistive layer lacks interstitial voidsbetween the coated ceramic particles and is non-porous.

The following abbreviations have the indicated meanings in thisdisclosure:

CMC=carboxymethyl cellulose

MCMB=mesocarbon microbeads

NMC=Nickel, Manganese and Cobalt

NMP=N-methylpyrrolidone

PTC=positive temperature coefficient

PVDF=polyvinylidene fluoride

SBR=styrene butadiene rubber

Torlon®4000TF=neat resin polyamide-imide (PAI) fine powder

Preparation of the resistance layer and electrode active layer isdescribed below, along with battery cell assembly.

The following is a generalized procedure for preparing a resistancelayer.

-   -   i. Dissolve the binder into an appropriate solvent.    -   ii. Add the conductive additive and ceramic powder into the        binder solution to form a slurry.    -   iii. Coat the slurry made in Step ii. onto the surface of a        metal foil, and then dry it to form a resistance layer on the        surface of the foil.

The following is a generalized procedure for the electrode preparation(on the top of the first layer).

-   -   iv. Dissolve the binder into an appropriate solvent.    -   v. Add the conductive additive into the binder solution to form        a slurry.    -   vi. Put the cathode or anode material into the slurry made in        the Step v. and mix it to form the slurry for the electrode        coating.    -   vii. Coat the electrode slurry made in the Step vi. onto the        surface of the layer from Step iii.    -   viii. Compress the electrode into the design thickness.

The following is a generalized procedure for Cell assembly.

-   -   ix. Dry the positive electrode at 125° C. for 10 hours and        negative electrode at 140° C. for 10 hours.    -   x. Punch the electrodes into the pieces with the electrode tab.    -   xi. Laminate the positive and negative electrodes with the        separator as the middle layer.    -   xii. Put the flat jelly-roll made in the Step xi. into an        aluminum composite bag.

Below are the generalized steps for conducting an impact test, as shownin FIG. 13, for a battery cell with a resistance layer.

-   -   i. Charge the cell at 2 A and 4.2V for 3 hours.    -   ii. Put the cell onto a hard flat surface such as concrete.    -   iii. Attach a thermal couple to the surface of the cell with        high temperature tape and connect the positive and negative tabs        to the voltage meter.    -   iv. Place a steel rod (15.8 mm±0.1 mm in diameter by about 70 mm        long) on its side across the center of the cell.    -   v. Suspend a 9.1±0.46 Kg steel block (75 mm in diameter by 290        mm high) at a height of 610±25 mm above the cell.    -   vi. Using a containment tube (8 cm inside diameter) to guide the        steel block, release the steel block through the tube and allow        it to free fall onto the steel bar laying on the surface of the        cell causing the separator to breach while recording the        temperature.    -   vii. Leave the steel rod and steel block on the surface of the        cell until the cell temperature stabilizes near room        temperature.    -   viii. End test.

Below are the generalized steps for testing a battery cell with aresistance layer for cycle life.

-   -   i. Rest for 5 minutes.    -   ii. Discharge to 2.8V.    -   iii. Rest for 20 minutes.    -   iv. Charge to 4.2V at 0.7 A for 270 minutes.    -   v. Rest for 10 minutes.    -   vi. Discharge to 2.8V at 0.7 A.    -   vii. Rest for 10 minutes.    -   viii. Repeat Steps iii to vii 100 times.    -   ix. End test.

The overcharge test generally follows the protocol below.

-   -   i. Charge the cell at 2 A and 4.2V for 3 hours.    -   ii. Put the charged cell into a room temperature oven.    -   iii. Connect the cell to a power supply (manufactured by        Hewlett-Packard).    -   iv. Set the voltage and current on the power supply to 12V and 2        A.    -   v. Turn on the power supply to start the overcharge test while        recording the temperature and voltage.    -   vi. Test ends when the cell temperature decreases and stabilizes        near room temperature.

Resistance Measurement Test protocol is as follows.

-   -   i. Place one squared copper foil (4.2×2.8 cm) with the tab on to        a metal plate (˜12×˜8 cm). Then cut a piece of thermal tape and        carefully cover one side of the squared copper foil.    -   ii. Cut a piece of the electrode that is slightly larger than        the copper paper. Place the electrode on to the copper foil.    -   iii. Place another copper foil (4.2×2.8 cm) with tab on the        electrode surface, repeat steps i-ii with it.    -   iv. At this point, carefully put them together and cover them        using high temperature tape and get rid of any air bubble    -   v. Cut a “V” shaped piece of metal off both tabs.    -   vi. Attach the completed strip to the metal clamp and tighten        the screws. Make sure the screws are really tight.    -   vii. Attach the tabs to the connectors of Battery HiTester        (produced by Hioki USA Corp.) to measure the resistance to make        sure that a good sample has been made for the measurement.    -   viii. Put the metal clamp inside the oven, connect the “V”        shaped tabs to the connectors and then tightened the screw. Tape        the thermal couple onto the metal clamp.    -   ix. Attach the Battery HiTester to the wires from oven. Do not        mix up the positive and the negative wires.    -   x. Close the oven and set the temperature to 200° C. at 4° C.        per minute, and start the test. Record data every 15 seconds.    -   xi. Stop recording the data when the metal clamp and oven reach        just a little over 200° C.    -   xii. Turn off the oven and the Battery HiTester.    -   xiii. End Test.

The Cycle Life procedure includes the following.

-   -   i. Rest for 5 minutes.    -   ii. Discharge to 2.8V.    -   iii. Rest for 20 minutes.    -   iv. Charge to 4.2V at 0.7 A for 270 minutes.    -   v. Rest for 10 minutes.    -   vi. Discharge to 2.8V at 0.7 A.    -   vii. Rest for 10 minutes.    -   viii. Repeat Steps iii to vii 100 times.

i. End Test.

Below are the generalized steps for testing a battery cell with aresistance layer for discharge at 1 A, 3 A, 6 A, and 10 A. In each test,the battery cell is tested in a chamber with controlled, constanttemperature, for example 50° C.

-   -   i. Rest for 5 minutes.    -   ii. Discharge to 2.8V.    -   iii. Rest for 20 minutes.    -   iv. Charge to 4.2V at 0.7 A for 270 minutes.    -   v. Rest for 10 minutes.    -   vi. Discharge to 2.8V at 1 A.    -   vii. Rest for 10 minutes.    -   viii. Charge to 4.2V at 0.7 A for 270 minutes.    -   ix. Rest for 10 minutes.    -   x. Discharge to 2.8V at 3 A.    -   xi. Charge to 4.2V at 0.7 A for 270 minutes.    -   xii. Rest for 10 minutes.    -   xiii. Discharge to 2.8V at 6 A.    -   xiv. Charge to 4.2V at 0.7 A for 270 minutes.    -   xv. Rest for 10 minutes.    -   xvi. Discharge to 2.8V at 10 A.    -   xvii. Rest for 10 minutes.    -   xviii. End Test.

As used herein, “high energy density rechargeable (HEDR) battery” meansa battery capable of storing relatively large amounts of electricalenergy per unit weight on the order of about 50 W-hr/kg or greater andis designed for reuse, and is capable of being recharged after repeateduses. Non-limiting examples of HEDR batteries include metal-ionbatteries and metallic batteries.

As used herein, “metal-ion batteries” means any rechargeable batterytypes in which metal ions move from the negative electrode to thepositive electrode during discharge and back when charging. Non-limitingexamples of metal-ion batteries include lithium-ion, aluminum-ion,potassium-ion, sodium-ion, magnesium-ion, and the like.

As used herein, “metallic batteries” means any rechargeable batterytypes in which the anode is a metal or metal alloy. The anode can besolid or liquid. Metal ions move from the negative electrode to thepositive electrode during discharge and back when charging. Non-limitingexamples of metallic batteries include M-S, M-NiCl₂, M-V₂O₅, M-Ag₂VP₂O₈,M-TiS₂, M-TiO₂, M-MnO₂, M-MO₃S₄, M-MoS₆Se₂, M-MoS₂, M-MgCoSiO₄,M-Mg_(1.03)Mn_(0.97)SiO₄, and others, where M=Li, Na, K, Mg, Al, or Zn.

As used herein, “lithium-ion battery” means any rechargeable batterytypes in which lithium ions move from the negative electrode to thepositive electrode during discharge and back when charging. Non-limitingexamples of lithium-ion batteries include lithium cobalt oxide (LiCoO₂),lithium iron phosphate (LiFePO₄), lithium manganese oxide (LiMn₂O₄),lithium nickel oxide (LiNiO₂), lithium nickel manganese cobalt oxide(LiNiMnCoO₂), lithium nickel cobalt aluminum oxide (LiNiCoAlO₂), lithiumtitanate (Li₄Ti₅O₁₂), lithium titanium dioxide, lithium/graphene,lithium/graphene oxide coated sulfur, lithium-sulfur, lithium-purpurin,and others. Lithium-ion batteries can also come with a variety of anodesincluding silicon-carbon nanocomposite anodes and the like. Lithium-ionbatteries can be in various shapes including small cylindrical (solidbody without terminals), large cylindrical (solid body with largethreaded terminals), prismatic (semi-hard plastic case with largethreaded terminals), and pouch (soft, flat body). Lithium polymerbatteries can be in a soft package or pouch. The electrolytes in thesebatteries can be a liquid electrolyte (such as carbonate based orionic), a solid electrolyte, a polymer based electrolyte or a mixture ofthese electrolytes.

As used herein, “aluminum-ion battery” means any rechargeable batterytypes in which aluminum ions move from the negative electrode to thepositive electrode during discharge and back when charging. Non-limitingexamples of aluminum-ion batteries include Al_(n)M₂(XO₄)₃, wherein X=Si,P, S, Mo, As, and others; and M=Fe, Ca, Mg, V, Cr and the like; aluminumtransition-metal oxides (Al_(x)MO₂ wherein M=Fe, Mn, Ni, Mo, Co, Cr, Ti,V and others) such as Al_(x)(V₄O₈), Al_(x)NiS₂, Al_(x)FeS₂, Al_(x)VS₂and Al_(x)WS₂ and the like.

As used herein, “potassium-ion battery” means any rechargeable batterytypes in which potassium ions move from the negative electrode to thepositive electrode during discharge and back when charging. Non-limitingexamples of potassium-ion batteries include K_(n)M₂(XO₄)₃, wherein X=Si,P, S, Mo, As, and others; and M=Fe, Ca, Mg, V, Cr and others; potassiumtransition-metal oxides (KMO₂ wherein M=Fe, Mn, Ni, Mo, Co, Cr, Ti, Vand others), and the like.

As used herein, “sodium-ion battery” means any rechargeable batterytypes in which sodium ions move from the negative electrode to thepositive electrode during discharge and back when charging. Non-limitingexamples of sodium-ion batteries include Na_(n)M₂(XO₄)₃, wherein X=Si,P, S, Mo, As, and others; and M=Fe, Ca, Mg, V, Cr and others;NaV_(1−x)Cr_(x)PO₄F, NaVPO₄F, Na₄Fe₃(PO₄)₂(P₂O₇), Na₂FePO₄F, Na₂FeP₂O₇,Na_(2/3) [Fe_(1/2)Mn_(1/2)]O₂, Na(Ni_(1/3)Fe_(1/3)Mn_(1/3))O₂, NaTiS₂,NaFeF₃; sodium transition-metal oxides (NaMO₂ wherein M=Fe, Mn, Ni, Mo,Co, Cr, Ti, V and others) such as Na_(2/3)[Fe_(1/2)Mn_(2/3)]O₂,Na(Ni_(1/3)Fe_(1/3)Mn_(1/3))O₂, Na_(x)MO₂O₄, NaFeO₂, Na_(0.7)CoO₂,NaCrO₂, NaMnO₂, Na_(0.44)MnO₂, Na_(0.7)MnO₂, Na_(0.7)MnO_(2.25),Na_(2/3)Mn_(2/3)Ni_(1/3)O₂, Na_(0.61)Ti_(0.48)Mn_(0.52)O₂; vanadiumoxides such as Na_(1+x)V₃O₈, Na_(x)V₂O₅, and Na_(x)VO₂ (x=0.7, 1); andthe like.

As used herein, “magnesium-ion battery” means any rechargeable batterytypes in which magnesium ions move from the negative electrode to thepositive electrode during discharge and back when charging. Non-limitingexamples of magnesium-ion batteries include Mg_(n)M₂(XO₄)₃, whereinX=Si, P, S, Mo, As, and others; and M=Fe, Ca, Mg, V, Cr and others;magnesium transition-metal oxides (MgMO₂ wherein M=Fe, Mn, Ni, Mo, Co,Cr, Ti, V and others), and the like.

As used herein, “binder” means any material that provides mechanicaladhesion and ductility with inexhaustible tolerance of large volumechange. Non-limiting examples of binders include styrene butadienerubber (SBR)-based binders, polyvinylidene fluoride (PVDF)-basedbinders, carboxymethyl cellulose (CMC)-based binders, poly(acrylic acid)(PAA)-based binders, polyvinyl acids (PVA)-based binders,poly(vinylpyrrolidone) (PVP)-based binders, poly carbonate, and polyethylene oxide and the like.

As used herein, “conductive additive” means any substance that increasesthe conductivity of the material. Non-limiting examples of conductiveadditives include carbon black additives, graphite non-aqueous ultrafinecarbon (UFC) suspensions, carbon nanotube composite (CNT) additives(single and multi-wall), carbon nano-onion (CNO) additives,graphene-based additives, reduced graphene oxide (rGO), conductiveacetylene black (AB), conductive poly(3-methylthiophene) (PMT),filamentary nickel powder additives, aluminum powder, someelectrochemically active oxides such as lithium nickel cobalt managesoxide, and the like.

As used herein, “metal foil” means any metal foil that under highvoltage is stable. Non-limiting examples of metal foils include aluminumfoil, copper foil, titanium foil, steel foil, nano-carbon paper,graphene paper, carbon fiber sheet, and the like.

As used herein, “ceramic powder” means any electrical insulator orelectrical conductor that has not been fired into a sintered, solidbody. Non-limiting examples of ceramic powder materials include bariumtitanate (BaTiO₃), zirconium barium titanate, strontium titanate(SrTiO₃), calcium titanate (CaTiO₃), magnesium titanate (MgTiO₃),calcium magnesium titanate, zinc titanate (ZnTiO₃), lanthanum titanate(LaTiO₃), and neodymium titanate (Nd₂Ti₂O₇), barium zirconate (BaZrO₃),calcium zirconate (CaZrO₃), lead magnesium niobate, lead zinc niobate,lithium niobate (LiNbO₃), barium stannate (BaSnO₃), calcium stannate(CaSnO₃), magnesium aluminum silicate, sodium silicate (NaSiO₃),magnesium silicate (MgSiO₃), barium tantalate (BaTa₂O₆), niobium oxide,zirconium tin titanate, and the like.

In some embodiments, layers can be coated onto metal foils by anautomatic coating machine (e.g. a compact coater, such as model number3R250W-2D produced by Thank-Metal Co., Ltd.). Layers can then becompressed to the desired thickness using a machine with rollers, forexample a calender machine (e.g., model number X15-300-1-DZ produced byBeijing Sevenstar Huachuang Electronics Co., Ltd.).

EXAMPLES

The high energy density rechargeable (HEDR) battery with a resistivelayer is described more in detail below using examples, but the battery,battery cell, or methods of making or using the battery are not limitedto the examples below.

Example 1

Preparation of baseline, positive and negative electrodes, and thecompleted Cell 1 for the evaluation in the resistance measurement,discharge capability tests at 50° C., impact test, and cycle life testare described below.

A) Preparation of POS1 A as an Example of the Positive ElectrodePreparation.

i) PVDF (21.6 g) was dissolved into NMP (250 g); ii) Carbon black (18 g)was added and mixed for 15 min at 6500 rpm; iii)LiNi_(0.5)Mn_(0.3)CO_(0.2)O₂ (NMC) (560.4 g) was added to the slurryfrom Step ii and mixed for 30 min at 6500 rpm to form a flowable slurry;iv) Some NMP was added for the viscosity adjustment; v) This slurry wascoated onto 15 μm aluminum foil using an automatic coating machine withthe first heat zone set to about 80° C. and the second heat zone toabout 130° C. to evaporate off the NMP. The final dried solid loadingwas about 15.55 mg/cm². The positive layer was then compressed to athickness of about 117 μm. This electrode was designated as zero voltageagainst a standard graphite electrode and was used for the impedancemeasurement at 0 V in relation to the temperature, and was used for thedry portion of the cell assembly.

B) Preparation of NEG2 A as an Example of the Negative ElectrodePreparation

i) CMC (5.2 g) was dissolved into deionized water (˜300 g); ii) Carbonblack was added (8.4 g) and mixed for 15 min at 6500 rpm; iii) Negativeactive graphite (JFE Chemical Corporation; Graphitized Mesophase CarbonMicro Bead (MCMB) and Synthetic Graphite (TIMCAL), 378.4 g in total, wasadded to the slurry from Step ii and mixed for 30 min at 6500 rpm toform a flowable slurry; iv) SBR (solid content 50% suspended in water)(16.8 g) was added to the slurry formed in Step iii and mixed at 6500rpm for 5 min; v) The viscosity was adjusted for a smooth coating; vi)This slurry was coated onto 9 μm thick copper foil using an automaticcoating machine with the first heat zone set to about 70° C. and thesecond heat zone to about 100° C. to evaporate off the water. The finaldried solid loading was about 9.14 mg/cm². The negative electrode layerwas then compressed to a thickness of about 117 μm. This negativeelectrode was used for the dry for the cell assembly.

C) Preparation of Cell for the Evaluation

i) The electrodes were punched into the pieces with an electrode tab;ii) The positive electrode was dried at 125° C. for 10 hours and thenegative electrode at 140° C. for 10 hours; iii) The positive andnegative electrodes were laminated with the separator as the middlelayer; iv) The flat jelly-roll made in the Step iii. was laid flat intoan aluminum composite bag; v) The bag from Step iv. was dried in a 70°C. vacuum oven; vi) The bag from Step v was filled with the LiPF₆containing organic carbonate based electrolyte; vii) The bag from Stepvi was sealed; viii) Rested for 16 hours; ix) The cell was charged to4.2V at C/20 rate for 5 hours and then to 4.2V at 0.5 C rate for 2hours, then rested for 20 minutes, then discharged to 2.8V at 0.5 Crate. Under vacuum, the cell was punctured to release any gases and thenresealed. The cell made here was used for grading and other tests suchas discharging capability test at 50° C., impact test, cycle life testand so on.

FIG. 9 presents the resistance in relation to the temperature increasefor the positive electrode collected from autopsying a cell with 3.6 V.The resistance decreases about ten times. FIG. 11 shows the dischargecapacity at the discharging currents 1, 3, 6, 10 A. FIG. 8 lists thecell impedance at 1 kHz and the capacity at 1 A, 3 A, 6 A and 10 Acurrents and the ratio of the capacity at 3, 6, 10 A over that at 1 A.FIG. 14 shows the cell temperature profile during the impact test. FIG.15 summarizes the cell maximum temperature in the impact test. The cellcaught fire during the impact test.

Example 2

Preparation of Al₂O₃ based resistive layer, positive and negativeelectrodes, and the completed Cell 3 for the evaluation in theresistance measurement, discharge capability tests at 50° C., impacttest, and cycle life test are described below.

A) Positive POS3B as an Example of a Resistance Layer (1^(st) Layer)Preparation.

i) Dissolve Torlon®4000TF (1 g) into NMP (10 g); Dissolve PVDF (6 g)into NMP (70 g); iii) Mix solution prepared in Step i and ii, and thenadd carbon black (0.4 g) and mix for 10 min at 6500 rpm; iv) Add nanoAl₂O₃ powder (42 g) to the solution from Step iii and mix for 20 min atthe rate of 6500 rpm to form a flowable slurry; v) Coat this slurry onto15 μm thick aluminum foil using an automatic coating machine with thefirst heat zone set to about 130° C. and the second heat zone to about160° C. to evaporate off the NMP. The final dried solid loading is about1 mg/cm².

B) Preparation of POS3 A as an Example of the Positive ElectrodePreparation (2nd Layer).

i) PVDF (21.6 g) was dissolved into NMP (250 g); Carbon black (18 g) wasadded and mixed for 15 min at 6500 rpm; LiNi_(1/3)CO_(1/3)Mn_(1/3)O₂(NMC) (560.4 g) was added to the slurry from Step ii and mixed for 30min at 6500 rpm to form a flowable slurry; iv) Some NMP was added forthe viscosity adjustment; v) This slurry was coated onto POS3B (Example2A) using an automatic coating machine with the first heat zone set toabout 85° C. and the second heat zone to about 135° C. to evaporate offthe NMP. The final dried solid loading was about 19.4 mg/cm². Thepositive layer was then compressed to a thickness of about 153 μm. Theelectrode made here is called as zero voltage against a standardgraphite electrode and was used for the impedance measurement at 0 V inrelation to the temperature.

C) Preparation of NEG3 A as an Example of the Negative ElectrodePreparation

i) CMC (13 g) was dissolved into deionized water (1000 g); ii) Carbonblack (20 g) was added and mixed for 15 min at 6500 rpm; iii) Negativeactive graphite (JFE Chemical Corporation; Graphitized Mesophase CarbonMicro Bead (MCMB) and Synthetic Graphite (TIMCAL) (945.92 g in total)was added to the slurry from Step ii and mixed for 30 min at 6500 rpm toform a flowable slurry; iv) SBR (solid content 50% suspended in water)(42 g) was added to the slurry formed in Step iii and mixed at 6500 rpmfor 5 min; v) The viscosity was adjusted for a smooth coating; vi) Thisslurry was coated onto 9 gm thick copper foil using an automatic coatingmachine with the first heat zone set to about 100° C. and the secondheat zone to about 130° C. to evaporate off the water. The final driedsolid loading was about 11.8 mg/cm². The negative electrode layer wasthen compressed to a thickness of about 159 μm. The negative made wasused for the dry for the cell assembly.

D) Preparation of Cell for the Evaluation

i) The electrodes were punched into the pieces with the electrode tab;ii) The positive electrode was dried at 125° C. for 10 hours andnegative electrode at 140° C. for 10 hours; iii) The positive andnegative electrodes were laminated with the separator as the middlelayer; iv) The flat jelly-roll made in the Step iii. was laid flat intoan aluminum composite bag; v) The bag from Step iv. was dried in a 70°C. vacuum oven; vi) The bag from Step v was filled with the LiPF₆containing organic carbonate based electrolyte; vii) The bag from Stepvi was sealed; viii) Rested for 16 hours; ix) The cell was charged to4.2V at C/50 rate for 8 hours and then to 4.2V at 0.5 C rate for 2hours, then rest for 20 minutes, then discharged to 2.8V at 0.5 C rate.Under vacuum, the cell was punctured to release any gases and thenresealed. The cell made here was used for grading and other tests suchas discharging capability test at 50° C., impact test, cycle life testand so on.

FIG. 10 presents the resistance in relation to the temperature increasefor the positive electrode collected from autopsying a cell with 4.09V.The resistance changes very little compared with that (FIG. 9) of thebaseline cell. FIG. 17 shows the discharge capacity vs. the cyclenumber. The cell lost about 2% capacity that is similar to that (2.5%)of the baseline cell. FIG. 8 lists the cell impedance at 1 kHz and thecapacity at 1 A, 3 A, 6 A and 10 A currents and the ratio of thecapacity at 3 A, 6 A, 10 A over that at 1 A. FIG. 14 shows the celltemperature profiles during the impact test. FIG. 15 summarizes the cellmaximum temperature in the impact test.

Example 3

Preparation of 50% Polyacrylic latex and 50% Barium Tatanate (BaTiO2)based resistive layer, positive and negative electrodes, and thecompleted Cell 4 for the evaluation in the resistance measurement,discharge capability tests at 50° C., impact test, and cycle life testare described below.

A) Positive POS4B as an Example of a Resistance Layer (1^(st) Layer)Preparation.

i) CMC (0.375 g) was dissolved into deionized water (˜30 g); ii) Thesolution prepared in Step i was mixed, and then carbon black (1.75 g)was added and mixed for several minutes; iii) nano BaTiO₂ powder (25 g)was added to the solution from Step ii and mixed for 20 min at 6500 rpmto form a flowable slurry; v) This slurry was coated onto 15 μm thickaluminum foil using an automatic coating machine with the first heatzone set to about 90° C. and the second heat zone to about 140° C. toevaporate off the water. The final dried solid loading was about 0.7mg/cm².

B) Preparation of POS4 A as an Example of the Positive ElectrodePreparation (2^(nd) Layer).

i) PVDF (14.4 g) was dissolved into NMP (˜160 g); ii) Carbon black (12g) was added and mixed for 15 min at 6500 rpm; iii)LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ (NMC) (373.6 g) was added to the slurryfrom Step ii and mixed for 30 min at 6500 rpm to form a flowable slurry;iv) Some NMP was added for the viscosity adjustment; v) This slurry wascoated onto POS4B (Example 2A) using an automatic coating machine withthe first heat zone set to about 80° C. and the second heat zone toabout 130° C. to evaporate off the NMP. The final dried solid loadingwas about 15.2 mg/cm². The positive layer was then compressed to athickness of about 113 μm. The electrode made here was called as zerovoltage against a standard graphite electrode and was used for theimpedance measurement at 0 V in relation to the temperature.

C) Preparation of NEG3 A as an Example of the Negative ElectrodePreparation

i) CMC (7.8 g) was dissolved into deionized water (˜800 g); ii) Carbonblack (12 g) was added and mixed for 15 min at 6500 rpm; iii) Negativeactive graphite (JFE Chemical Corporation; Graphitized Mesophase CarbonMicro Bead (MCMB) and Synthetic Graphite (TIMCAL) (568.6 g in total) wasadded to the slurry from Step ii and mixed for 30 min at 6500 rpm toform a flowable slurry; iv) SBR (solid content 50% suspended in water)(25.2 g) was added to the slurry formed in Step iii and mixed at 6500rpm for 5 min; v) The viscosity was adjusted for a smooth coating; vi)This slurry was coated onto 9 μm thick copper foil using an automaticcoating machine with the first heat zone set to about 70° C. and thesecond heat zone to about 100° C. to evaporate off the water. The finaldried solid loading was about 8.99 mg/cm². The negative electrode layerwas then compressed to a thickness of about 123 μm. The negative madewas used for the dry for the cell assembly.

D) Preparation of Cell for the Evaluation

i) The electrodes were punched into the pieces with an electrode tab;ii) The positive electrode was dried at 125° C. for 10 hours andnegative electrode at 140° C. for 10 hours; iii) The positive andnegative electrodes were laminated with the separator as the middlelayer; iv) The flat jelly-roll made in the Step iii. was laid flat intothe aluminum composite bag; v) The bag from Step iv. was dried in a 70°C. vacuum oven; vi) The bag from Step v was filled with the LiPF₆containing organic carbonate based electrolyte; vii) The bag from Stepvi was sealed; viii) Rest for 16 hours; ix) The cell was charged to 4.2Vat C/50 rate for 8 hours and then to 4.2V at 0.5 C rate for 2 hours,then rest for 20 minutes, then discharged to 2.8V at 0.5 C rate. Undervacuum, the cell was punctured to release any gases and then resealed.The cell made here was used for grading and other tests such asdischarging capability test at 50° C., impact test, cycle life test andso on.

FIG. 12 shows the discharge capacity at 1 A, 3 A, 6 A current and at 50°C. The cell capacity decreases very rapidly with the increase of thecurrent, indicating the strong effect from the resistive layer. FIG. 8lists the cell impedance at 1 kHz and the capacity at 1 A, 3 A, 6 A and10 A currents and the ratio of the capacity at 3 A, 6 A, 10 A over thatat 1 A. FIG. 14 shows the cell temperature profiles during the impacttest. FIG. 15 summarizes the cell maximum temperature in the impacttest.

Example 4

Preparation of resistive layer in negative electrodes, positive andnegative electrodes, and the completed Cell 5 for the evaluation in theresistance measurement, discharge capability tests at 50° C., impacttest, and cycle life test are described below.

A) Preparation of POS5 A as an Example of the Positive ElectrodePreparation.

i) PVDF (31.5 g) was dissolved into NMP (˜340 g); ii) Carbon black (13.5g) was added and mixed for 15 min at 6500 rpm; iii)LiNi_(0.33)Mn_(0.33)CO_(0.33)O₂ (NMC) (855 g) was added to the slurryfrom Step ii and mix for 30 min at 6500 rpm to form a flowable slurry;iv) Some NMP was added for the viscosity adjustment; v) This slurry wasadded onto 15 μm aluminum foil using an automatic coating machine withthe first heat zone set to about 80° C. and the second heat zone toabout 130° C. to evaporate off the NMP. The final dried solid loadingwas about 14.8 mg/cm². The positive layer was then compressed to athickness of about 113 μm. The electrode made here was designated aszero voltage against a standard graphite electrode and was used for theimpedance measurement at 0 V in relation to the temperature, and the dryfor the cell assembly.

B) Preparation of NEG5B as an Example of the Negative ElectrodePreparation (1^(st) Layer)

CMC (0.375 g) was dissolved into deionized water (˜90 g); ii) Carbonblack (1.75 g) was added and mixed for 15 min; BaTiO₂ (25 g in total)was added to the slurry from Step ii and mixed for 30 min at 6500 rpm toform a flowable slurry; iv) SBR (solid content 50% suspended in water)(45.6 g) was added to the slurry formed in Step iii and mixed at about6500 rpm for 5 min; v) The viscosity was adjusted for a smooth coating;vi) This slurry was coated onto 9 μm thick copper foil using anautomatic coating machine with the first heat zone set to about 90° C.and the second heat zone to about 140° C. to evaporate off the water.

C) Preparation of NEG5 A as an Example of the Negative ElectrodePreparation (2^(nd) Layer)

i) CMC (3.9 g) was dissolved into deionized water (˜350 g); ii) Carbonblack (6 g) was added and mixed for 15 min at 6500 rpm; iii) Negativeactive graphite (JFE Chemical Corporation; Graphitized Mesophase CarbonMicro Bead (MCMB) and Synthetic Graphite (TIMCAL) (283.8 g in total)were added to the slurry from Step ii and mixed for 30 min at 6500 rpmto form a flowable slurry; iv) SBR (solid content 50% suspended inwater) (25.2 g) was added to the slurry formed in Step iii and mixed at6500 rpm for 5 min; v) The viscosity was adjusted for a smooth coating;vi) This slurry was coated onto NEG5B (Example 4B) using an automaticcoating machine with the first heat zone set to about 70° C. and thesecond heat zone to about 100° C. to evaporate off the water. The finaldried solid loading was about 9.8 mg/cm². The negative electrode layerwas then compressed to a thickness of about 114 μm. The negative madewas used for the dry for the cell assembly.

D) Preparation of Cell for the evaluation

i) The electrodes were punched into the pieces with the electrode tab;ii) The positive electrode was dried at 125° C. for 10 hours andnegative electrode at 140° C. for 10 hours; iii) The positive andnegative electrodes were laminated with the separator as the middlelayer; iv) The flat jelly-roll made in the Step iii. was laid flat intothe aluminum composite bag; v) The bag from Step iv. was dried in a 70°C. vacuum oven; vi) The bag from Step v was filled with the LiPF₆containing organic carbonate based electrolyte; vii) The bag from Stepvi was sealed; viii) Rest for 16 hours; ix) The cell was charged to 4.2Vat C/20 rate for 5 hours and then to 4.2V at 0.5 C rate for 2 hours,then rest for 20 minutes, then discharged to 2.8V at 0.5 C rate. Undervacuum, the cell was punctured to release any gases and then resealed.The cell made here was used for grading and other tests such asdischarging capability test at 50° C., impact test, cycle life test andso on.

FIG. 8 lists the cell impedance at 1 kHz and the capacity at 1 A, 3 A, 6A and 10 A currents and the ratio of the capacity at 3 A, 6 A, 10 A overthat at 1 A. FIG. 14 shows the cell temperature profile during theimpact test. FIG. 15 summarizes the cell maximum temperature in theimpact test.

Example 5

Preparation of Al₂O₃ and Sodium trisilicate (NaSiO₃) mixed basedresistive layer, positive and negative electrodes, and the completedCell 6 for the evaluation in the resistance measurement, dischargecapability tests at 50° C., impact test, and cycle life test aredescribed below.

A) Positive POS6B as an Example of a Resistance Layer (1^(st) Layer)Preparation.

i) Torlon®4000TF (0.8 g) was dissolved into NMP (˜10 g); ii) PVDF (4.8g) was dissolved into NMP (60 g); iii) The solutions prepared in Step iand ii were mixed, and then carbon black (0.32 g) was added and mixedfor 10 min at 6500 rpm; iv) nano Al₂O₃ powder (17.04 g) and NaSiO₃(17.04 g) were added to the solution from Step iii and mixed for 20 minat 6500 rpm to form a flowable slurry; v) This slurry was coated onto 15μm thick aluminum foil using an automatic coating machine with the firstheat zone set to about 135° C. and the second heat zone to about 165° C.to evaporate off the NMP. The final dried solid loading was about 0.7mg/cm².

B) Preparation of POS6 A as an Example of the Positive ElectrodePreparation (2^(nd) Layer).

i) PVDF (21.6 g) was dissolved into NMP (270 g); ii) Carbon black (18 g)was added and mixed for 15 min at 6500 rpm; iii)LiNi_(1/3)CO_(1/3)Mn_(1/3)O₂ (NMC) (560.4 g) was added to the slurryfrom Step ii and mixed for 30 min at 6500 rpm to form a flowable slurry;iv) Some NMP was added for the viscosity adjustment; v) This slurry wascoated onto POS6B (Example 1A) using an automatic coating machine withthe first heat zone set to about 85° C. and the second heat zone toabout 135° C. to evaporate off the NMP. The final dried solid loadingwas about 19.4 mg/cm². The positive layer was then compressed to athickness of about 153 μm. The electrode made here was called as zerovoltage against a standard graphite electrode and was used for theimpedance measurement at 0 V in relation to the temperature.

C) Preparation of NEG6 A as an Example of the Negative ElectrodePreparation

i) CMC (13 g) was dissolved into deionized water (˜1000 g); ii) Carbonblack (20 g) was added and mixed for 15 min at 6500 rpm; iii) Negativeactive graphite (JFE Chemical Corporation; Graphitized Mesophase CarbonMicro Bead (MCMB) and Synthetic Graphite (TIMCAL) (945.92 g in total)were added to the slurry from Step ii and mix for 30 min at 6500 rpm toform a flowable slurry; iv) SBR (solid content 50% suspended in water)(42 g) was added to the slurry formed in Step iii and mixed at 6500 rpmfor 5 min; v) The viscosity was adjusted for a smooth coating; vi) Thisslurry was coated onto 9 μm thick copper foil using an automatic coatingmachine with the first heat zone set to about 100° C. and the secondheat zone to about 130° C. to evaporate off the water. The final driedsolid loading was about 11.8 mg/cm². The negative electrode layer wasthen compressed to a thickness of about 159 μm. The negative made isready for the dry for the cell assembly.

D) Preparation of Cell for the Evaluation

i) The electrodes were punched into the pieces with an electrode tab;ii) The positive electrode was dried at 125° C. for 10 hours andnegative electrode at 140° C. for 10 hours; iii) The positive andnegative electrodes were laminated with the separator as the middlelayer; iv) The flat jelly-roll made in the Step iii. was put into analuminum composite bag; v) The bag from Step iv. was dried in a 70° C.vacuum oven; vi) The bag from Step v was filled with the LiPF₆containing organic carbonate based electrolyte; vii) The bag from Stepvi was sealed; viii) Rest for 16 hours; ix) The cell was charged to 4.2Vat C/50 rate for 8 hours and then to 4.2V at 0.5 C rate for 2 hours,then rest for 20 minutes, then discharged to 2.8V at 0.5 C rate. Undervacuum, the cell was punctured to release any gases and then resealed.The cell made here was used for grading and other tests such asdischarging capability test at 50° C., impact test, cycle life test andso on.

FIG. 8 lists the cell impedance at 1 kHz and the capacity at 1 A, 3 A, 6A and 10 A currents and the ratio of the capacity at 3 A, 6 A, 10 A overthat at 1 A. FIG. 14 shows the cell temperature profiles during theimpact test. FIG. 15 summarizes the cell maximum temperature in theimpact test.

Example 6

Preparation of CaCO₃ based gas generator layer, positive and negativeelectrodes, and the cell (#7) for the evaluation in the over charge testis described below. This gas generator layer could become a resistivelayer if the conductive additive content is in the certain range suchthat the resistivity of the gas-generator layer is more resistive (50%more at least) than that of the energy layer or the layer that providethe majority (>50%) of the battery discharge energy. The gas generatorcontent can be 2% to 99%.

A) Positive POS071 A as an Example of a Gas Generator Layer (1^(st)Layer) Preparation.

i) Torlon®4000TF (0.9 g) was dissolved into NMP (10 g); ii) PVDF (5.25g) was dissolved into NMP (˜68 g); iii) The solutions prepared in Step iand ii were mixed, and then carbon black (1.8 g) was added and mixed for10 min at the rate of about 6500 rpm; iv) Nano CaCO₃ powder (7.11 g) and134.94 g LiNi_(0.33)Mn_(0.33)CO_(0.33)O₂ were added to the solution fromStep iii and mixed for 20 min at the rate of about 6500 rpm to form aflowable slurry; v) This slurry was coated onto 15 μm thick aluminumfoil using an automatic coating machine with the first heat zone set toabout 90° C. and the second heat zone to about 140° C. to evaporate offthe NMP. The final dried solid loading was about 4 mg/cm².

B) Preparation of POS071B as an Example of the Positive ElectrodePreparation (2nd Layer).

i) PVDF (25.2 g) was dissolved into NMP (327 g); ii) Carbon black (21 g)was added and mixed for 15 min at the rate of about 6500 rpm; iii)LiNi_(0.82)Al_(0.03)Co_(0.15)O₂ (NCA) (649 g) was added to the slurryfrom Step ii and mixed for 30 min at the rate of about 6500 rpm to forma flowable slurry; iv) Some NMP was added for the viscosity adjustment;v) This slurry was coated onto POS071 A using an automatic coatingmachine with the first heat zone set to about 85° C. and the second heatzone to about 135° C. to evaporate off the NMP. The final dried solidloading is about 20.4 mg/cm². The positive layer was then compressed toa thickness of about 155 μm.

C) Preparation of NEG015B as an Example of the Negative ElectrodePreparation

i) CMC (15 g) was dissolved into deionized water (˜951 g); ii) Carbonblack (15 g) was added and mixed for 15 min at the rate of about 6500rpm; iii) Negative active graphite (JFE Chemical Corporation;Graphitized Mesophase Carbon Micro Bead (MCMB) (945 g) was added to theslurry from Step ii and mixed for 30 min at the rate of about 6500 rpmto form a flowable slurry; iv) SBR (solid content 50% suspended inwater) (50 g) was added to the slurry formed in Step iii and mixed atabout 6500 rpm for 5 min; v) The viscosity was adjusted for a smoothcoating; vi) This slurry was coated onto 9 μm thick copper foil using anautomatic coating machine with the first heat zone set to about 100° C.and the second heat zone to about 130° C. to evaporate off the water.The final dried solid loading was about 11 mg/cm². The negativeelectrode layer was then compressed to a thickness of about 155 μm. Thenegative made is ready for the dry for the cell assembly.

D) Preparation of Cell for the Evaluation

i) The electrodes were punched into the pieces with an electrode tab;ii) The positive electrode was dried at ˜125° C. for 10 hr and negativeelectrode at ˜140° C. for 10 hr; iii) The positive and negativeelectrodes were laminated with the separator as the middle layer; iv)The flat jelly-roll made in the Step iii was laid flat in an aluminumcomposite bag; v) The bag from Step iv. was dried in a 70° C. vacuumoven; vi) The bag from Step v was filled with the carbonate basedelectrolyte; vii) The bag from Step vi was sealed; viii) Rest for 16hours; ix) The cell was charged to 4.2V at C/50 rate for 8 hours andthen to 4.2V at 0.5 C rate for 2 hours, then rest for 20 minutes, thendischarged to 2.8V at 0.5 C rate. The cell made here was used forgrading and other tests such as over charge test.

FIG. 18 presents the overcharge voltage, cell temperature and ovenchamber temperature during the overcharge test (2 A and 12V). The cellpassed the over test nicely since the cell maximum temperature is about83° C. during the overcharge test. Implementations of the currentsubject matter can include, but are not limited to, articles ofmanufacture (e.g. apparatuses, systems, etc.), methods of making or use,compositions of matter, or the like consistent with the descriptionsprovided herein.

In the descriptions above and in the claims, phrases such as “at leastone of” or “one or more of” may occur followed by a conjunctive list ofelements or features. The term “and/or” may also occur in a list of twoor more elements or features. Unless otherwise implicitly or explicitlycontradicted by the context in which it used, such a phrase is intendedto mean any of the listed elements or features individually or any ofthe recited elements or features in combination with any of the otherrecited elements or features. For example, the phrases “at least one ofA and B;” “one or more of A and B;” and “A and/or B” are each intendedto mean “A alone, B alone, or A and B together.” A similarinterpretation is also intended for lists including three or more items.For example, the phrases “at least one of A, B, and C;” “one or more ofA, B, and C;” and “A, B, and/or C” are each intended to mean “A alone, Balone, C alone, A and B together, A and C together, B and C together, orA and B and C together.” Use of the term “based on,” above and in theclaims is intended to mean, “based at least in part on,” such that anunrecited feature or element is also permissible.

The subject matter described herein can be embodied in systems,apparatus, methods, and/or articles depending on the desiredconfiguration. The implementations set forth in the foregoingdescription do not represent all implementations consistent with thesubject matter described herein. Instead, they are merely some examplesconsistent with aspects related to the described subject matter.Although a few variations have been described in detail above, othermodifications or additions are possible. In particular, further featuresand/or variations can be provided in addition to those set forth herein.For example, the implementations described above can be directed tovarious combinations and subcombinations of the disclosed featuresand/or combinations and subcombinations of several further featuresdisclosed above. In addition, the logic flows depicted in theaccompanying figures and/or described herein do not necessarily requirethe particular order shown, or sequential order, to achieve desirableresults. Other implementations may be within the scope of the followingclaims.

1. A high energy density rechargeable (HEDR) battery comprising: ananode energy layer; a cathode energy layer; a separator between theanode energy layer and the cathode energy layer for preventing internaldischarge thereof; at least one current collector for transferringelectrons to and from either the anode or cathode energy layer, theanode and cathode energy layers each having an internal resistivity, theHEDR battery having a preferred temperature range for dischargingelectric current and an upper temperature safety limit; and a resistivelayer interposed between the separator and one of the currentcollectors, the resistive layer configured to limit the rate of internaldischarge through the separator in the event of separator failure andthe generation of joule heat resulting therefrom, the resistive layerhaving a fixed resistivity at temperatures between the preferredtemperature range and the upper temperature safety limit, the fixedresistivity of the resistive layer being greater than the internalresistivity of either energy layer, the resistive layer for avoidingtemperatures in excess of the upper temperature safety limit in theevent of separator failure.
 2. The HEDR battery of claim 1 wherein theresistive layer is porous and comprises: a ceramic powder defining aninterstitial space; a binder for partially filling the interstitialspace for binding the ceramic powder; and a conductive componentdispersed within the binder for imparting resistivity to the resistivelayer, the interstitial space remaining partially unfilled for impartingporosity and permeability to the resistive layer.
 3. The HEDR battery ofclaim 2 wherein the resistive layer is compressed to reduce the unfilledinterstitial space and increase the binding of the ceramic powder by thebinder.
 4. The HEDR battery of claim 2 wherein the resistive layercomprises greater than 30% ceramic powder by weight. 5.-8. (canceled) 9.The HEDR battery of claim 2 wherein the resistive layer is permeable totransport of ionic charge carriers.
 10. The HEDR battery of claim 1wherein the resistive layer is non-porous and has a compositioncomprising: a non-conductive filler; a binder for binding thenon-conductive filler; and a conductive component dispersed within thebinder for imparting resistivity to the resistive layer.
 11. The HEDRbattery of claim 10 wherein the resistive layer is impermeable totransport of ionic charge carriers.
 12. The HEDR battery of claim 1wherein the fixed resistivity of the resistive layer is at least twiceas great as the internal resistivity of either energy layer. 13.(canceled)
 14. (canceled)
 15. The HEDR battery of claim 1 wherein theresistive layer lacks a physical phase transformation at temperaturesbetween the preferred temperature range and the upper temperature safetylimit for transforming the resistivity of the resistive layer.
 16. TheHEDR battery of claim 15 wherein the resistive layer lacks atransformation from solid phase to non-solid phase for transforming theresistivity of the resistive layer from low resistivity to highresistivity at temperatures between the maximum operating temperatureand the upper temperature safety limit.
 17. The HEDR battery of claim 1wherein the resistive layer is non-sacrificial at temperatures below theupper temperature safety limit.
 18. The HEDR battery of claim 17 whereinthe resistive layer is sacrificial at temperatures above the uppertemperature safety limit.
 19. The HEDR battery of claim 18 wherein theresistive layer comprises a ceramic powder that chemically decomposesabove the upper temperature safety limit for evolving a fire retardantgas and/or a gas for delaminating the current collector from theresistive layer.
 20. (canceled)
 21. The HEDR battery of claim 1 of atype wherein the current collector comprises an anode current collectorfor transferring electrons to and from the anode energy layer, whereinthe resistive layer is interposed between the separator and the anodecurrent collector.
 22. The HEDR battery of claim 21, wherein theresistive layer is interposed between the anode current collector andthe anode energy layer.
 23. The HEDR battery of claim 21, wherein theresistive layer is interposed between the anode energy layer and theseparator.
 24. The HEDR battery of claim 21 wherein the anode energylayer comprises: a first anode energy layer; and a second anode energylayer interposed between the first anode energy and the separator,wherein the resistive layer is interposed between the first anode energylayer and the second anode energy layer.
 25. The HEDR battery of claim 1wherein the current collector comprises a cathode current collector fortransferring electrons to and from the cathode energy layer, wherein theresistive layer is interposed between the separator and the cathodecurrent collector.
 26. The HEDR battery of claim 25, wherein theresistive layer is interposed between the cathode current collector andthe cathode energy layer.
 27. The HEDR battery of claim 25, wherein theresistive layer is interposed between the cathode energy layer and theseparator.
 28. The HEDR battery of claim 25 wherein the cathode energylayer comprises: a first cathode energy layer; and a second cathodeenergy layer interposed between the first cathode energy and theseparator, wherein the resistive layer is interposed between the firstcathode energy layer and the second cathode energy layer.
 29. The HEDRbattery of claim 1 further comprising two current collectors comprising:an anode current collector for transferring electrons to and from theanode energy layer; and a cathode current collector for transferringelectrons to and from the cathode energy layer, wherein the resistivelayer comprises an anode resistive layer and a cathode resistive layer,the anode resistive layer interposed between the separator and the anodecurrent collector, the cathode resistive layer interposed between theseparator and the cathode current collector.
 30. A method for limitingthe rate of an internal discharge of energy layers resulting from aseparator failure within a high energy density rechargeable (HEDR)battery, the method comprising: resisting the internal discharge with aresistive layer, the resistive layer being interposed between aseparator and a current collector within the HEDR battery, the resistivelayer having a fixed resistivity at temperatures between a preferredtemperature range for discharging the energy layers and an uppertemperature safety limit, the fixed resistivity of the resistive layerbeing greater than the internal resistivity of the energy layers.